This rule came about after a boiler explosion on the Gettysburg railway associated with a plugged water glass.
NTSB Summary: [www.ntsb.gov]


You can read the entire FRA rule for Steam Locomotives by searching for "49 CFR Part 230" as follows:
[ecfr.gpoaccess.gov]

Here is an excerpt dealing with 15-year inspections

§ 230.17 One thousand four hundred seventy-two (1472) service day inspection.
(a) General. Before any steam locomotive is initially put in service or brought out of retirement, and after every 1472 service days or 15 years, whichever is earlier, an individual competent to conduct the inspection shall inspect the entire boiler

Because few steam locomotives are in continuous operation, FRA discusses service days as follows.


§ 230.18 Service days.
(a) Service day record. For every steam locomotive currently in service, the steam locomotive owner and/or operator shall have available, and be able to show an FRA inspector upon request, a current copy of the service day record that contains the number of service days the steam locomotive has accrued since the last 31, 92, Annual and 1472 service day inspections.

(b) Service day report. By the 31st of every January, every steam locomotive owner and/or operator shall file a service day report, FRA Form No. 5, with the Regional Administrator accounting for the days the steam locomotive was in service from January 1 through December 31st of the preceding year. If the steam locomotive was in service zero (0) days during that period, a report must still be filed to prevent the steam locomotive from being considered retired by FRA. (See appendix B of this part.)

(c) Retirement where no service day reports filed. Where the steam locomotive owner and/or operator does not file the required service day report for a steam locomotive, that steam locomotive may be considered retired by FRA. The steam locomotive owner and/or operator must complete all 1472 service day inspection items to return a retired steam locomotive to service.